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Contact tracing: Trying to get it right

Vet Advice with Dr. Ron Clarke

Contact tracing is not limited to infectious disease epidemics among humans; it is also used during infectious disease outbreaks in livestock.

Epidemiologic contact tracing is an arduous and time-consuming process, yet very important. Timely interventions reduce the size and scale of infectious disease epidemics. Methods that are more efficient at identifying contacts allow more effective controls to be implemented sooner, reducing the epidemic’s magnitude. Ebola epidemics in West Africa and a two-year COVID-19 pandemic that infected over 200 million worldwide and killed over four million demonstrate the danger of committing too few resources to contact tracing soon enough to be maximally effective (Science Direct). Chronic wasting disease in Canadian elk and deer herds and failure to find the source of bovine tuberculosis involved in Canadian beef herds are just two other recent examples of traceability letdowns.

Contact tracing is not limited to infectious disease epidemics in human populations; it is extensively used during infectious disease outbreaks of farmed animals. When animal epidemics threaten industry sectors, and animal movement determines its reach, we gain an appreciation for the resources required for contract tracing, along with where to focus surveillance. Playing into this are factors such as animal identification, farm identification, management of those data at markets and ultimately co-ordination of activity between livestock sectors. These elements increase the efficacy of control efforts by ensuring timely investigation or quarantine of at-risk farms. Allocating sufficient resources to the task is crucial to successful contact tracing.

The rapid and frequent movement of animals between farms is a prominent characteristic of modern animal agriculture throughout the world. Infected animals not obviously sick at the time of shipment can infect other animals on farms that are hundreds of miles from the source farm. Newly infected farms present a risk to other susceptible farms and represent underlying reason for pre-entry isolation/quarantine related to domestic sales, and international health certification for export sales. In the end, contact tracing in animal agriculture tracks potential infection sources and where they might have gone. Farms identified as contacts frequently undergo quarantine (depending on the disease), allowing them to be monitored until free of disease.

Recent experience with foot-and mouth-disease and highly pathogenic avian influenza epidemics has heightened interest in — and demonstrated the use of — various mapping and data management systems to optimize surveillance strategies and epidemic control in farmed animals. Most of this work has been carried out in regions where animal movements are monitored and the location and nature of agricultural enterprises a matter of public record. Canada continues to refine what is in place, while the U.S. struggles to build a compulsory, national animal identification system that supports tracking animal movement. Inadequacies of the U.S. system necessitate the creation of synthetic farm maps and human-made renditions of animal movement networks. Tracing contact farms is simplified by the voluntary registration of farm addresses and production details in three-dimensional databases like the United States Animal Health Emergency Reporting Diagnostic System database (USAHERDS). The Pennsylvania USAHERDS database is unique in its comprehensiveness. It can be used to create a network showing the pattern of movement of captive deer between farms in Pennsylvania.

Proposed changes to Canada’s Livestock Identification and Traceability Program aim to reduce the time it takes to trace-in and trace-out herds from weeks to days by using animal identification, premises identification and movement information entered into the responsible administrator’s database. This would improve data accuracy and availability in the event of a disease outbreak or an emergency. The goal of the Canadian Food Inspection Agency’s proposed regulatory amendments is to address the gaps in the current livestock identification system previously identified during consultations in 2013 and 2015, including:

  • Livestock species that share diseases are not all subject to traceability requirements.
  • The time allowed to report an event to a responsible administrator is too long to support an efficient response to disease outbreaks, or natural disaster.
  • Information about the geographical location of sites (premises) where animals are located is limited.
  • Information about the domestic movements of livestock is unknown or not readily available.

Traceability is important because it provides timely, accurate and relevant information to reduce the effects of a disease outbreak, food safety issue or natural disasters originating from and/or affecting livestock. While traceability is critical to protecting animal health, it provides strong marketing paybacks for Canadian meat products domestically and in export markets.

Traceability also plays a key role in programs that are in place to meet consumers’ expectations for food safety, animal welfare and sustainability, such as the Canadian Cattlemen’s Association Verified Beef Production Plus (VBP+) and the Dairy Farmers of Canada’s proAction initiative.

The first step to any investigation is to determine the identification number of an affected animal, and any movements made by that animal. This allows investigators to identify the locations where the animal may have interacted with other animals, helping to identify the source and contain the disease. Without identification and movement information, it is extremely difficult to pinpoint the specific animals and herds that may have been affected.

For example, an outbreak of bovine tuberculosis in Alberta and Saskatchewan in November 2016 resulted in tracing back animal movements for the previous five years to determine all animals that might have been infected. As a result of inadequate animal identification across many herds and the lack of animal movement information, more than 160 herds (56,000 animals) were quarantined until testing could be completed.

About the author


Dr. Ron Clarke

Dr. Ron Clarke prepares this column on behalf of the Western Canadian Association of Bovine Practitioners. Suggestions for future articles can be sent to Canadian Cattlemen ([email protected]) or WCABP ([email protected]).



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